Devices which measure and count particles in a fluid are well known. Such devices are employed, for example, by semiconductor wafer manufacturers to monitor the extent of airborne particulate matter in a clean room. Pharmaceutical manufacturers employ such devices for the detection and control of foreign particles. To a lesser extent of accuracy, smoke detectors also measure particle concentration.
One method of particle detection is the light blockage particle counting, or light obscuration, method. Light obscuration sensors work on the principle of the casting of a shadow onto a photodetector as a flow of particle-laden fluid is directed through a light beam generated by an incandescent lamp. A more sensitive method is the light scattering method. As a particle passes through a light beam, the particle scatters light. For a stationary particle, the amount of scattered light is a function of the particle size, the wavelength and intensity of the incident light, and the difference between the light scattering properties of the particle and the surrounding medium. A laser source may be used to generate the light beam and the scattered light is sensed by a detector which provides readable signals indicative of particle size.
In addition to those factors listed above, which enter into the determination of the amount of light scattered by a particle, other factors must be considered where the particle is not stationary but rather is contained within a sample flow of fluid. To detect all particles in a sample flow, the flow must have a cross section sufficiently small to remain completely within the view volume of a detection device. In applications such as clean room monitoring, the flow rate of a given volume is typically a standard, for example, one cubic foot per minute. Consequently, the minimum velocity of the particle-laden sample flow is fixed by the view volume of the incident beam.
The velocity of a sample flow determines the time in which a particle remains within the view volume of the detection device. This time is important for two reasons. Firstly, the quantity of light which a given particle can scatter while in the view volume is inversely proportional to particle velocity and directly proportional to particle size. Increasing the velocity of a particle by a factor of two results in a halving of the time span in which the particle travels through the view volume, thereby decreasing the quantity of scattered light by half.
Secondly, the pulse width plays an important part in optimizing the signal-to-noise ratio of a particle detector. The minimum particle velocity fixes the minimum time in which a particle travels through the view volume of the incident beam, which, in turn, governs the minimum pulse width which is produced by the detector electronics. Fast pulses require the electronics high frequency response corner to increase. Raising the high frequency response corner increases the amount of amplifier noise, thus degrading the signal-to-noise ratio of the particle counter. However, since particle velocity through a view volume is typically high, fast pulses are produced, so that the high frequency noise is less likely to be rejected.
One answer to increasing the aggregate amount of light scattered by a particle while in the view volume and simultaneously raise the minimum expected pulse width, is to reduce the velocity of the sample flow and spread out the sample time. However, sample flows of a given volume are typically specified by the industry.
Another possible answer is to increase the view volume of the device. The maximum view volume, however, is limited by the light source and the scattered light collection optics.
It is an object of the present invention to provide an apparatus for detecting particles, wherein the aggregate amount of light scattered by a given particle in a sample volume is significantly increased, and wherein the minimum expected pulse width caused by a particle traveling through a view volume is likewise increased.